I personally do not have yellow teeth, but definitely do not have noticeably white ones, either. But, if you ask me, I definitely think that a white smile is one of the sexiest things a girl can have. It just screams natural and healthy which in the long-run are more important than tan and glamorous.
At the moment, I'm currently using crest whitestrips advanced seal (I'll post pictures when I'm finished the pack). These suckers are great and really stick to your teeth. I honestly think at $35 these are a steal since you only need to reapply every 6 months, but if you aren't down to shell out a few tens, fear not. I've seriously come up with something genius...and you'll only need to buy two things (under $10 total!).
So the first thing you'll need is a mouthguard like the one pictured above. At walmart you can snag one of these guys for under 3 dollars, trust. You'll actually want two though. So here's the deal. While my BFF spent all of her time in highschool saving up for custom whitening trays, I spent my time playing contact sports. Consequently, we ended up with the same thing: A clear plastic thingy for your teeth. Only difference was mine cost like one millionth of the price. You boil water and put a mouthguard in it for about ten seconds, just enough to see if soften up and lose shape a little, then take it out (use tongs!) and put it in cold water for just a moment. Then, stick the guard in your mouth and, well, suck! get all the water out and press the plastic to your teeth as you bite down. After doing that for about 5-10 seconds repeat but mold the guard to your bottom teeth. Taaada! Custom trays.